1. Field of the Disclosure
The present disclosure relates to a chemically prepared toner formulation for use in electrophotography, and more specifically, to a toner formulation having a crystalline polyester that is encapsulated by a styrene acrylate latex formulation and method of preparing the same. The disclosed method of preparing the toner results in a change in the distribution of the components of the toner particle wherein the lower molecular weight resins, the pigment, and the wax are located away from the surface of the toner particle.
2. Description of the Related Art
Toners for use in electrophotographic printers include two primary types, mechanically milled toners and chemically prepared toners (CPT). Chemically prepared toners have significant advantages over mechanically milled toners including better print quality, higher toner transfer efficiency and lower torque properties for various components of the electrophotographic printer such as a developer roller, a fuser belt and a charge roller. The particle size distribution of CPTs is typically narrower than the particle size distribution of mechanically milled toners. The size and shape of CPTs are also easier to control than mechanically milled toners. There are several known types of CPT including suspension polymerization toner (SPT), emulsion aggregation toner (EAT)/latex aggregation toner (LAT), toner made from a dispersion of pre-formed polymer in solvent (DPPT) and “chemically milled” toner. While emulsion aggregation toner requires a more complex process than other CPTs, the resulting toner has a relatively narrower size distribution. Emulsion aggregation toners can also be manufactured with a smaller particle size allowing improved print resolution. The emulsion aggregation process also permits better control of the shape and structure of the toner particles that allows them to be tailored to fit the desired cleaning, doctoring and transfer properties. The shape of the toner particles may be optimized to ensure proper and efficient cleaning of the toner from various electrophotographic printer components, such as the developer roller, charge roller and doctoring blades, in order to prevent filming or unwanted deposition of toner on these components.
In a typical process for preparing EAT, emulsion aggregation is carried out in an aqueous system resulting in good control of both the size and shape of the toner particles. The toner components typically include a polymer binder, one or more colorants and a release agent. A styrene acrylic copolymer polymer binder can be used as the latex binder in the emulsion aggregation process. However, the use of a styrene acrylic copolymer latex binder requires a tradeoff between the fusing properties and shipping and storage properties of the toner. The fusing properties of the toner include its fuse window. The fuse window is the range of temperatures at which fusing is satisfactorily conducted without incomplete fusion and without transfer of toner to the heating element, which may be a roller, belt or other member contacting the toner during fusing. Thus, below the low end of the fuse window, the toner is incompletely melted and above the high end of the fuse window the toner flows onto the fixing member where it mars subsequent sheets being fixed. It is preferred that the low end of the fuse window be as low as possible to reduce the required temperature of the fuser in the electrophotographic printer to conserve energy. However, the toner must also be able to survive the temperature and humidity extremes associated with storage and shipping without caking or blocking which may result in print flaws. As a result, the low end of the fuse window cannot be so low that the ship store property of the toner is unacceptable, thereby melting the toner contained in the toner cartridge during shipping and storage.
Toners formed from polyester binder resins can possess better mechanical properties than toners formed from a styrene acrylic copolymer binder of similar melt viscosity characteristics, thereby making them more durable and resistant to filming of printer components. Polyester toners also have better compatibility with color pigments resulting in a wider color gamut. However, the use of polyester binder resins in toners also has limitations such as increased expense to manufacture and limiting the fusing properties of the toner. Additionally, polyester binder resins are more difficult to disperse in an aqueous system due to their polar nature, pH sensitivity and gel content thereby limiting their applicability in the emulsion aggregation process.
The inventors of the present invention believe it is possible to cost effectively produce a toner that also has the desirable low energy fusing temperature and does not degrade during shipping or storage. This is achieved by combining the advantages of both styrene acrylate and polyester resins in the toner particle. However, it is often difficult to combine these two resins because it is difficult to anchor the styrene acrylic onto the polyester resin particles in the EA toner manufacturing process, especially when the toner is formulated into a core shell structure. Another problem that arises in chemically prepared styrene acrylate, polyester, and hybrid resin based toners is the migration of waxes, lower molecular weight resins, such as crystalline polyester and short chain styrene acrylate polymer, and colorants to the surface of the toner particle. The migration of these components to the surface of the toner particle weakens the fusing and ship/store properties of the toner, and increases the occurrence of filming on printer components. Prior art methods to make chemically prepared core shell toner do not completely prevent the migration of such components to the surface of the toner particle. It would be advantageous for the toner to have the lower molecular weight resins, the pigment, and the wax to be located away from the surface of the toner particle. Moreover, a very desirous distribution of the wax and pigment in the toner particle is for the wax to accumulate into larger domains located away from the surface of the toner particle and for the pigment to accumulate on the edges of these large wax domains. This particular distribution improves the fusing, charging, ship/store properties of the toner and controls the color-to-color variation between different colored toners.
Accordingly, there is a need for an emulsion aggregation toner formulation and process that reduces the migration of lower molecular weight resins, waxes, and colorants to the surface of the toner particle. It would be desirable for the toner to have the pigment and the wax to be located away from the surface of the toner particle. Moreover, a very desirous distribution of the wax and pigment in the toner particle is for the wax to accumulate into larger domains located away from the surface of the toner particle and for the pigment to accumulate on the edges of these large wax domains. The disclosed method of preparing the toner results in this desirable distribution of the components of the toner. This desirable change in the distribution of these components in the toner particle is accomplished by first encapsulating a crystalline polyester with a styrene acrylic latex and then adding this encapsulated crystalline polyester latex to the remaining components in the toner in an emulsion aggregation process. These particular steps performed in an emulsion aggregation process surprisingly changes the distribution of the components in the toner particle, wherein the wax accumulates into larger domains located away from the surface of the toner particle and the pigment accumulates on the edges of these large wax domains. Without wishing to be bound by theory, it is believed that the functional groups in the styrene acrylic latex act as an anchor for the pigment, which in turn positively influences the pigment distribution in the toner particles. This particular arrangement reduces the likelihood of the styrene acrylic, crystalline polyester, wax or pigment migrating to the toner surface, thereby reducing the likelihood of weakening the fusing and ship/store properties of the toner, and the occurrence of filming on printer components.